The invention to methods of hyperpolarizing noble gases. More particularly, the invention relates to methods of high efficiency optical pumping methods for hyperpolarizing noble gases.
It is known that noble gases such as .sup.3 He and .sup.129 Xe can be "hyperpolarized" using laser techniques. Such polarization methods include spin-exchange optical pumping, by which an alkali metal vapor is optically polarized, followed by "exchange" of this polarization with the noble gas (Bouchiat et al. 1960; Bhaskar et al. 1982; Happer et al. 1984; Zeng et al. 1985; Cates et al. 1992). Other polarization methods employ metastability exchange, in which noble gas nuclei (typically helium-3 (.sup.3 He)) are directly optically pumped without an alkali metal intermediary (Schearer 1969; Laloe et al. 1984). Systems for producing polarized noble gases are described in U.S. Pat. Nos. 5,642,625 and 5,617,860, the complete disclosures of which are incorporated herein by reference.
Hyperpolarized noble gases can be used for numerous purposes. Historically, polarized .sup.129 Xe has been used for fundamental symmetry studies (Chupp et al. 1994), nuclear spin relaxation studies of solids (Gatzke et al. 1993), high resolution nuclear magnetic resonance spectroscopy (NMR) (Raftery et al. 1991), and cross-polarization to other nuclei (Gatzke et al. 1993; Long et al. 1993). Polarized .sup.3 He is also an important nuclear target (Anthony et al. 1993; Middleton (1994)).
Most recently, the enhanced NMR signals of laser polarized .sup.129 Xe, which are about five orders of magnitude larger than those from thermally polarized .sup.129 Xe, have made possible the first high-speed biological magnetic resonance imaging (MRI) of a gas (Albert et al. 1994). Helium-3 has also proven to be an excellent nucleus for gas phase MRI (Middleton et al. 1995). U.S. Pat. No. 5,545,396 describes the use of .sup.129 Xe, .sup.3 He, and other noble gas nuclei for biological MRI. These striking advances are now opening many new avenues of research.
The principal limitation in these applications of polarized noble gases has been the availability of sufficient quantities of the gases to meet the demand. Accordingly, attention has been directed to improving the rates of polarized noble gas production. Apparatus has been devised by which larger quantities of polarized gas can be produced on a continuous or batch mode basis. See U.S. Pat. No. 5,642,645. Methods for limiting depolarization of noble gases by interactions with container surfaces have been addressed by providing polymers as coatings. U.S. Pat. No. 5,612,103. Apparatus has also been developed to permit storage of frozen polarized .sup.129 Xe. See U.S. application Ser. No. 08/622,865, filed on Mar. 29, 1996, the complete disclosure of which is incorporated herein by reference.
Even with these advances, the processes by which noble gases can be polarized are capable of further improvement, as many parameters have not been optimized. For example, efficiency of polarization is limited by the physical properties of the materials used to construct the polarizing apparatus. Moreover, an incomplete understanding of theoretical considerations underlying the physics of spin exchange in various systems implies that opportunities exist to identify systems with greater efficiencies.
From a practical perspective, hyperpolarization efficiency is related to laser power, while the cost of laser installation and maintenance often increases directly with delivered power. Accordingly, polarization systems for producing higher amounts of polarized noble gases can require significantly more expensive lasers. Therefore, it would be desirable to enable the artisan to increase the polarization yield of a given laser, and thereby to mitigate expense in scaled-up systems.
Accordingly, it is one of the purposes of this invention to overcome the above limitations in the art of spin-exchange optical pumping methods, by providing methods by which polarization efficiently is significantly improved using currently available apparatus. It is another purpose of the invention to provide the artisan with materials and methods that enable a wider variety of apparatus useful for polarizing noble gases.